During a larval-pupal metamorphosis in a molometabolous insect, the corpora allata become inactive and do not secrete juvenile hormone (JH). In addition, the endogenous JH is inactivated before the larval insect cells can activate pupal sets of genes and metamorphose. It has been shown earlier that interactivation of endogenous JH is the critical step prior to induction of pupal synthesis in Galleria larval epidermis. The present study is directed to determine the time of appearance of JH metabolizing enzymes during this critical stage, with special reference to JH esterase. The tissue source of this enzyme and controls that regulate the enzyme levels will be investigated. Preliminary studies indicate that JH metabolizing enzymes may occur during a limited span in the last instar Galleria larvae. This investigation focuses on the role of JH, ecdysone, neurosecretory centers and JH-binding proteins in the control of JH esterase activity. A knowledge of levels of JH metabolizing enzymes is necessary for precise interpretation of the data on the response of insects to exogenous JH. Furthermore its control is the single most important factor in development of resistance to JH-based insecticides. Since insects are vectors of disease and feed on food and fibres needed by man the data obtained in the present study have practical value. With the knowledge of the mode of activation of JH esterase activity prior to pupation, it may be possible to control its synthesis and thus render the insect incapable of becoming resistant to the JH-based insecticides.